
SAB 82532/SAF 82532
HDLC/SDLC Serial Mode
Semiconductor Group
58
07.96
5.2.2
The LAP controllers of the two serial channels can be configured to function in a
half-duplex Normal Response Mode (NRM), where they operate as a slave (secondary)
station, by setting the NRM bit in the XBCH register of the corresponding channel.
In contrast to the full-duplex LAP B/LAP D operation, where the combined
(primary + secondary) station transmits both commands and responses and may
transmit data at any time, the NRM mode allows only responses to be transmitted
and
the secondary station may transmit only when instructed to do so by the master (primary)
station. The ESCC2 gets the permission to transmit from the primary station via an S-,
or I-frame with the poll bit (p)
set
.
The NRM mode can be profitably used in a point-to-multipoint configuration with a fixed
master-slave relationship, which guarantees the absence of collisions on the common
transmit line. It is the responsibility of the master station to poll the slaves periodically
and to handle error situations.
Prerequisite for NRM operation is:
– auto mode with 8-bit address field selected
MODE: MDS1, MDS0, ADM = ‘000’
– external timer mode
MODE: TMD = ‘0’
– same transmit and receive addresses, since only responses can be transmitted, i.e.
XAD1 = XAD2 = RAL1 = RAL2 (address of secondary).
Note: The broadcast address may be programmed in RAL2 if broadcasting is required.
In this case RAL1 and RAL2 are not equal.
Half-Duplex SDLC-NRM Operation
The primary station has to operate in transparent SDLC mode.